Chemical Looping Combustion of Rice Husk

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Chemical Looping Combustion of Rice Husk

Rashmeet Singh Monga

11

, Ganesh R. Kale

22

, Sadanand Y. Guhe

11

1 1

(Chemical Engineering Dept., Jawaharlal Nehru Engineering College,

(Chemical Engineering Dept., Jawaharlal Nehru Engineering College, Aurangabad, India)Aurangabad, India)

2 2

(Solid and Hazardous Waste Management Division, CSIR-NEERI, Nagpur, India) (Solid and Hazardous Waste Management Division, CSIR-NEERI, Nagpur, India)

ABSTRACT ABSTRACT

A thermodynamic investigation of direct chemical looping combustion (CLC) of rice husk is presented in this A thermodynamic investigation of direct chemical looping combustion (CLC) of rice husk is presented in this paper. Both

paper. Both steam and COsteam and CO22 are used for gasification within the temperature range of 500 are used for gasification within the temperature range of 500 – – 12001200˚˚C and differentC and different

amounts of oxygen carriers. Chemical equilibrium model was considered for the CLC fuel reactor.

amounts of oxygen carriers. Chemical equilibrium model was considered for the CLC fuel reactor. The trends inThe trends in product

product compositions compositions of of the the fuel fuel reactor, reactor, were were determined. determined. Rice Rice husk husk gasification gasification using using 3 3 moles moles HH22O and 0O and 0

moles CO

moles CO22 per mole carbon (in rice husk) at 1 bar pressure and 900 per mole carbon (in rice husk) at 1 bar pressure and 900 ˚˚C was found to be the best operating pointC was found to be the best operating point

for hundred percent carbon conversion in the fuel reactor. Such detailed thermodynamic studies can be useful to for hundred percent carbon conversion in the fuel reactor. Such detailed thermodynamic studies can be useful to design chemical looping combustion processes using different fuels.

design chemical looping combustion processes using different fuels. Key

Key WorWor ds ds - - Chemical Chemical looping combustion, NiO, Rice husk, Thermodynamic study. looping combustion, NiO, Rice husk, Thermodynamic study.

I.

I. IntroductionIntroduction

CO

CO22 emission to the environment is mainly emission to the environment is mainly

contributed by industrial power generation. Concerns contributed by industrial power generation. Concerns for climate change phenomenon have started special for climate change phenomenon have started special procedures

procedures to to reduce reduce COCO22 emissions using CO emissions using CO22

capture and storage techniques that capture CO

capture and storage techniques that capture CO22 from from

energy-intensive processes and then store it in energy-intensive processes and then store it in suitable geologic locations [1]. The concerns of suitable geologic locations [1]. The concerns of increase in greenhouse gas emissions and an increase in greenhouse gas emissions and an inevitable global warming crisis is reported by inevitable global warming crisis is reported by researchers. CO

researchers. CO22 is also largely generated by fossil is also largely generated by fossil

fuel processing and it is a crucial greenhouse gas that fuel processing and it is a crucial greenhouse gas that affects the climate change [2]. Fossil fuels are at affects the climate change [2]. Fossil fuels are at present

present the the major major energy resources energy resources and and are are likely likely toto dominate for more several decades. Hence it is dominate for more several decades. Hence it is essential to continue the use of fossil fuels but also essential to continue the use of fossil fuels but also reduce the CO

reduce the CO22 emissions to the atmosphere [3]. emissions to the atmosphere [3].

The expectations from the Carbon capture and The expectations from the Carbon capture and storage (CCS) technologies to help curb the storage (CCS) technologies to help curb the greenhouse gas emissions and ensure a sustainable greenhouse gas emissions and ensure a sustainable development of power generation and other development of power generation and other energy-intensive industrial sectors are extremely high. intensive industrial sectors are extremely high. Amongst them, chemical looping systems display a Amongst them, chemical looping systems display a promising option to

promising option to capture COcapture CO22 with lower cost and with lower cost and

energy penalty [1]. Chemical looping combustion energy penalty [1]. Chemical looping combustion (CLC) is a promising technology to utilize fossil fuel (CLC) is a promising technology to utilize fossil fuel for combustion and prevent CO

for combustion and prevent CO22 dilution in the dilution in the

nitrogen rich flue gases. In this process, a solid nitrogen rich flue gases. In this process, a solid oxygen carrier supplies the oxygen needed for CO oxygen carrier supplies the oxygen needed for CO22

and water formation that is a nitrogen free product and water formation that is a nitrogen free product mixture. This avoids the major cost of CO mixture. This avoids the major cost of CO22

separation from the flue gases and also reduces the separation from the flue gases and also reduces the NO

NO x x formation. A good oxygen carrier readily reactsformation. A good oxygen carrier readily reacts with the fuel gas and shall be reoxidized upon being with the fuel gas and shall be reoxidized upon being contacted with oxygen. An oxygen carrier is contacted with oxygen. An oxygen carrier is generally prepared by a metal oxide and an inert generally prepared by a metal oxide and an inert binder that

binder that provide tprovide the oxygen he oxygen storage, fluidizabilitystorage, fluidizability and mechanical strength [4]. Chemical looping and mechanical strength [4]. Chemical looping

combustion (CLC) is a non-flame two-step combustion (CLC) is a non-flame two-step combustion which produces a pure CO

combustion which produces a pure CO22 stream for stream for

easy compression and sequestration. The process easy compression and sequestration. The process comprises of two interconnected fluidized bed comprises of two interconnected fluidized bed reactors where the fuel reactor is a bubbling fluidized reactors where the fuel reactor is a bubbling fluidized bed

bed and and the the air air reactor reactor is is a a conventional cconventional c irculatingirculating fluidized bed.

fluidized bed. It uses It uses an oxygen an oxygen carrier - a carrier - a highly- highly-reactive metal particle, to avoid the direct contact of reactive metal particle, to avoid the direct contact of air and fuel during the combustion, to indirectly air and fuel during the combustion, to indirectly transport oxygen from the air to the fuel. The transport oxygen from the air to the fuel. The products

products of of combustion combustion are are kept kept separated separated from from thethe rest of the flue gases namely nitrogen and excess rest of the flue gases namely nitrogen and excess oxygen [5]. The CLC system is composed of two oxygen [5]. The CLC system is composed of two reactors, an air and a fuel reactor, as

reactors, an air and a fuel reactor, as shown in fig. 1shown in fig. 1

Fig. 1- Chemical-looping combustion system. Fig. 1- Chemical-looping combustion system.

RESEARCH

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In CLC, the solid oxygen carrier is circulated In CLC, the solid oxygen carrier is circulated between the air and

between the air and fuel reactors. The fuel reactors. The fuel is fed fuel is fed intointo the fuel reactor where it is oxidized by the lattice the fuel reactor where it is oxidized by the lattice oxygen of the oxygen carriers and the reaction in the oxygen of the oxygen carriers and the reaction in the fuel reactor is almost endothermic.

fuel reactor is almost endothermic. (2n +m) Me

(2n +m) MexxOOyy +C +CnnHH2m2m→→(2n + m) Me(2n + m) Me_x_xOO_y_y−−11 + +

mH

mH22O + nCOO + nCO22..

Once fuel oxidation is completed; the reduced Once fuel oxidation is completed; the reduced metal oxide M

metal oxide MyyOO(x(x−−1)1) is transported to the air reactor is transported to the air reactor

where it is reoxidized according the reaction: where it is reoxidized according the reaction:

Me

MexxOOyy−−11 + ½ O + ½ O22→→MeMexxOOyy

The flue gas stream from the air reactor will The flue gas stream from the air reactor will have a high temperature and contain N

have a high temperature and contain N22 and some and some

unreacted O unreacted O22..

Transition metal oxides, such as Ni, Fe, Cu, and Transition metal oxides, such as Ni, Fe, Cu, and Mn oxides are reported as reactive oxygen carriers. Mn oxides are reported as reactive oxygen carriers. Ni-based

Ni-based oxygen oxygen carriers carriers have have exhibited exhibited the the bestbest reactivity and stability during multi-redox cycles [6]. reactivity and stability during multi-redox cycles [6]. Henrik Leion et al. has investigated the feasibility of Henrik Leion et al. has investigated the feasibility of using three different solid fuels in chemical-looping using three different solid fuels in chemical-looping combustion (CLC) using NiO as oxygen carrier in a combustion (CLC) using NiO as oxygen carrier in a laboratory fluidized-bed reactor system at 970°C. laboratory fluidized-bed reactor system at 970°C. They reported that the NiO particles also showed They reported that the NiO particles also showed good reactivity with methane and a syngas mixture good reactivity with methane and a syngas mixture of 50% H

of 50% H22 and 50% CO also showed good fluidizing and 50% CO also showed good fluidizing

properties

properties without without any any signs signs of of agglomeration agglomeration [7].[7]. Tobias Mattisson et al. have investigated the Tobias Mattisson et al. have investigated the feasibility of using NiO as an oxygen carrier with feasibility of using NiO as an oxygen carrier with CH

CH44 as fuel and in the temperature range 700 as fuel and in the temperature range 700 – –

1200°C during chemical-looping combustion and 1200°C during chemical-looping combustion and their result showed that the yield of CO

their result showed that the yield of CO22 and H and H22OO

decreased with increase in temperature [8]. M.M. decreased with increase in temperature [8]. M.M. Yazdanpanah et al. has presented a reactor model for Yazdanpanah et al. has presented a reactor model for combustion of CH

combustion of CH44 with NiO/NiAl with NiO/NiAl22OO44 oxygen oxygen

carriers in the fuel reactor of a 10 kW CLC pilot carriers in the fuel reactor of a 10 kW CLC pilot plant.

plant. The The model model shows shows good good agreement agreement withwith experimental results considering some kinetic and experimental results considering some kinetic and hydrodynamic modifications, including impact of gas hydrodynamic modifications, including impact of gas adsorption by falling particles in the bubble phase on adsorption by falling particles in the bubble phase on the gas exchange coefficient and thermodynamic the gas exchange coefficient and thermodynamic limitation of CO and H

limitation of CO and H22 combustion [9]. C. Saha et combustion [9]. C. Saha et

al. have carried out an experimental investigation al. have carried out an experimental investigation pertaining

pertaining to to CLC CLC of of a a Victorian Victorian brown brown coal coal usingusing NiO and CuO as oxygen carriers at 950 °C (NiO) and NiO and CuO as oxygen carriers at 950 °C (NiO) and

800°C (CuO) and have reported the high

800°C (CuO) and have reported the high reactivity ofreactivity of CuO as compared to NiO during cyclic operation CuO as compared to NiO during cyclic operation [10]. Magnus Rydén et al. have

[10]. Magnus Rydén et al. have examined the Ce, Ca,examined the Ce, Ca, or Mg stabilized ZrO

or Mg stabilized ZrO22 oxygen carriers by redoxoxygen carriers by redox

experiments in a batch fluidized-bed reactor at 800 experiments in a batch fluidized-bed reactor at 800 – – 950°C, using CH

950°C, using CH44 as fuel. The experiments showed as fuel. The experiments showed

good reactivity between the particles and

good reactivity between the particles and CHCH44 [11]. [11].

Many studies have also demonstrated the Many studies have also demonstrated the feasibility of using CLC for both gaseous and solid feasibility of using CLC for both gaseous and solid fuels [12]. The chemical looping process was mainly fuels [12]. The chemical looping process was mainly

targeted towards efficient carbon capturing [13, 14] targeted towards efficient carbon capturing [13, 14] and hydrogen production [15, 16]. Rutuja Bhoje et and hydrogen production [15, 16]. Rutuja Bhoje et al. have done theoretical study of chemical looping al. have done theoretical study of chemical looping combustion of methane to consider some key combustion of methane to consider some key technology development points to help the process technology development points to help the process engineer choose the right oxygen carrier and process engineer choose the right oxygen carrier and process conditions [17]. X. Wang et al. have developed a conditions [17]. X. Wang et al. have developed a three-dimensional numerical model to simulate the three-dimensional numerical model to simulate the CLC process in the fuel reactor using a bubbling CLC process in the fuel reactor using a bubbling fluidized bed of oxygen carrier made from 14

fluidized bed of oxygen carrier made from 14 wt% ofwt% of metal oxide CuO and 86% wt of inert material Al metal oxide CuO and 86% wt of inert material Al22OO33

and a coal gas from coal gasification containing 55 and a coal gas from coal gasification containing 55 vol. % CO, 30 vol. % H

vol. % CO, 30 vol. % H22 and 15 vol. % CO and 15 vol. % CO22 [18]. [18].

India produces approximately 120 million tons India produces approximately 120 million tons of paddies each year which gives around 24 million of paddies each year which gives around 24 million tons of rice husk and 4.4

tons of rice husk and 4.4 million tons of rice husk ashmillion tons of rice husk ash every year. Rice husk is a high calorific value every year. Rice husk is a high calorific value renewable fuel and can be used for electricity renewable fuel and can be used for electricity generation in efficient manner [19]. Rice husk can be generation in efficient manner [19]. Rice husk can be used as fuel and has oxygen in its structure [20]. The used as fuel and has oxygen in its structure [20]. The CLC of rice husk has not been studied so far in CLC of rice husk has not been studied so far in literature.

literature.

II.

II. Process Design:Process Design:

The conceptual process design for CLC of rice

husk is shown in figure 2. The process scheme

consists of a CLC fuel reactor and air reactor.

Preheated rice husk, CO

Preheated rice husk, CO22, , HH22O and NiO from airO and NiO from air

reactor are fed to the fuel reactor in calculated

amount to convert the rice husk to CO

amount to convert the rice husk to CO22, and H, and H22O.O.

The fuel reactor products were assumed to be in

thermodynamic equilibrium at the exit of the reactor.

Complete conversion of rice husk and maximum CO

Complete conversion of rice husk and maximum CO22

production

production are are targeted targeted in in the the fuel fuel reactor. reactor. It It isis

assumed that the NiO oxidizes the carbon and

hydrogen in rice husk to CO

hydrogen in rice husk to CO22 and H and H22O in the CLCO in the CLC

fuel reactor. The moles of OC (NiO) for fuel reactor

are varied depending on the stoichiometric

requirement of reaction between rice husk and NiO.

Complete conversion of rice husk to CO

Complete conversion of rice husk to CO22 and H and H22O isO is

desired; however the conversion is limited by

thermodynamic equilibrium.

Fig. 2- Process diagram

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The OC (NiO) is reduced (to Ni)

The OC (NiO) is reduced (to Ni) in the CLC fuelin the CLC fuel

reactor. The products of the CLC fuel reactor go

through a gas-solid separator and the gaseous

products

products mostly mostly containing containing COCO22 and H and H22O can beO can be

cooled for CO

cooled for CO22 separation and sequestration with separation and sequestration with

heat recovery or recycled back as required. The solid

products

products of of the the CLC CLC fuel fuel reactor reactor containing Ocontaining OC C areare

transferred to the air reactor, in which preheated air

is added to completely oxidize coke to CO

is added to completely oxidize coke to CO22 and and

regenerate the reduced OC. Complete conversion is

assumed in these oxidation reactions.

The regenerated oxygen carrier (NiO) is

separated from the air reactor product stream using a

gas-solid separator and is recycled to the fuel reactor.

The air reactor is the major source of thermal energy

in this process. It is safely assumed that both the air

and fuel reactors operate at same temperature and

pressure conditions.

Table 1- Rice husk composition [21].

The base composition of rice husk taken were 1

mole C, 0.6402 moles hydrogen (H

mole C, 0.6402 moles hydrogen (H22), 0.3052 oxygen), 0.3052 oxygen

(O

(O22). According to rice husk composition reaction). According to rice husk composition reaction

involved in fuel reactor assumed to be

C+ 0.6402H

C+ 0.6402H22(g) + 0.3052O(g) + 0.3052O22 (g) + 2.0298NiO = CO (g) + 2.0298NiO = CO22

(g) + 0.6402H

(g) + 0.6402H22O O (g) (g) + + 2.0298Ni. 2.0298Ni. (1)(1)

III.

III. Process MethodologyProcess Methodology

::

Thermodynamic analysis of the system helps us Thermodynamic analysis of the system helps us to determine the optimum conditions that can to determine the optimum conditions that can maximize the yield with low energy consumption maximize the yield with low energy consumption [22].

[22]. HSC Chemistry software version 5.11 is used toHSC Chemistry software version 5.11 is used to

generate the thermodynamic equilibrium data for

CLC fuel reactor in this process study. The

thermodynamic equilibrium calculations in the Gibbs

routine of HSC Chemistry are done using the Gibbs

free energy minimization algorithm. The Gibbs

program

program searches searches the the best best combination combination of of mostmost

stable species in which the Gibbs free energy of the

system can achieve its minimum at a fixed mass

balance

balance (a (a constraint constraint minimization minimization problem),problem),

constant pressure, and temperature. Hence no

chemical reaction equations are required in the input.

The chemical species such as C(s), CO

The chemical species such as C(s), CO22 (g), H (g), H22

(g), CO (g), H

(g), CO (g), H22O (g), CHO (g), CH44 (g), H (g), H22O (l), OO (l), O22 (g), NiO (g), NiO

(s) and Ni (s), that are usually found in CLC reaction

system considered

system considered in this in this study. study. The input The input speciesspecies

fed to the fuel reactor were rice husk with oxygen

carrier, H

carrier, H22O (g), and COO (g), and CO22 (g). The material balances (g). The material balances

are done by Equilibrium Composition module of

HSC Chemistry and the results were used to

calculate reaction enthalpy by Reaction Equation

module of HSC Chemistry software.

Table 2- Fuel reactor feed condition with

Table 2- Fuel reactor feed condition with NiO in NiO in stoichiometric ratio.

stoichiometric ratio.

1 mol carbon (in rice husk) is used as basis for

all calculations in the temperature range of 500

all calculations in the temperature range of 500 – –

1200˚C for the entire process.

1200˚C for the entire process. The feed gasifying The feed gasifying

agent-to-carbon ratio (GaCR) ranging from 1 to 3

was used in this study. The CLC of rice husk using

both

both gasifying gasifying agent agent (CO(CO22 and H and H22O) and oxygenO) and oxygen

carrier (NiO) is studied with intermediate steps of

increase in CO

increase in CO22 moles (with simultaneous decrease moles (with simultaneous decrease

in steam moles) for constant GaCR ratios. These feed

conditions for CO

conditions for CO22 and steam input per mole carbon and steam input per mole carbon

(in rice husk) are shown in Table 2. These inputs

were used to calculate the thermodynamic

equilibrium composition in the CLC fuel reactor. T

equilibrium composition in the CLC fuel reactor. Thehe

oxidation reaction in the air reactor is exothermic.

It is presumed that, according to stoichiometric

amount given by the reactions the air is supplied to

the air reactor:

Ni + 0.5O

Ni + 0.5O22= = NiO NiO (2)(2)

C + O

C + O22 = CO = CO22 (3)(3)

IV.

IV. Results and Results and Discussions:Discussions:

4.1. Effect of Amount of oxygen carrier: 4.1. Effect of Amount of oxygen carrier:

The stoichiometric quantity of NiO was varied

according to fuel reactor reaction of oxygen carrier:

C + 0.6402H

C + 0.6402H22(g) + 0.3052 O(g) + 0.3052 O22 (g) + 2.0298 NiO = (g) + 2.0298 NiO =

CO

CO22 (g) +0.6402H (g) +0.6402H22O (g) + 2.0298 NiO (g) + 2.0298 Ni

Stoichiometric amount of NiO reqd. (S) = 2.0298

Assuming Assuming S S = = 2.02982.0298 Species Species C C HH22 OO22 Moles Moles 1 1 0.6402 0.6402 0.30520.3052

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1.25*S = 2.53725

And

And 1.5*S 1.5*S = = 3.04473.0447

The effect of stoichiometric amount of oxygen

carrier on CLC of rice husk was investigated for 1,

1.25, and 1.5 stoichiometric of NiO in fuel reactor.

Some side reactions also take place in the fuel Some side reactions also take place in the fuel reactor and hence the conversions in the fuel reactor reactor and hence the conversions in the fuel reactor are limited by thermodynamic equilibrium are limited by thermodynamic equilibrium constraints. It was necessary to study the equilibrium constraints. It was necessary to study the equilibrium product composition of

product composition of the CLC the CLC fuel reactor at fuel reactor at 1 bar1 bar pressure. Since, the OC is generally used in excess of pressure. Since, the OC is generally used in excess of the stoichiometric requirement in CLC processes to the stoichiometric requirement in CLC processes to enhance the syngas and CH

enhance the syngas and CH44 conversion in the fuel conversion in the fuel

reactor. Hence two more cases (1.25S and 1.5S) reactor. Hence two more cases (1.25S and 1.5S) using higher amounts (1.25 times and 1.5 times the using higher amounts (1.25 times and 1.5 times the stoichiometric requirement) of OC were also studied. stoichiometric requirement) of OC were also studied. 4.1.1. (CH

4.1.1. (CH44 + CO + H + CO + H22) formation:) formation:

The (CH

The (CH44 + CO + H + CO + H22) compositions in product) compositions in product

gases at 1 bar pressure and at 700°C of the CLC fuel gases at 1 bar pressure and at 700°C of the CLC fuel reactor for different inputs of OC and different input reactor for different inputs of OC and different input feed conditions are shown in Figure 3. It was feed conditions are shown in Figure 3. It was observed that the H

observed that the H22, CO, and CH, CO, and CH44 emissions from emissions from

the fuel reactor decreased with increase in the the fuel reactor decreased with increase in the amount of OC. At constant GaCR, it was also amount of OC. At constant GaCR, it was also observed that the (CH

observed that the (CH44 + CO + H + CO + H22) moles generally) moles generally

decreased with increase in feed CO

decreased with increase in feed CO22 moles (with moles (with

simultaneous decrease in H

simultaneous decrease in H22O moles). The maximumO moles). The maximum

amount of (CH

amount of (CH44 + CO + H + CO + H22) ) exit mexit moles woles were fere foundound

to be 0.19 moles for case CS1, while the minimum to be 0.19 moles for case CS1, while the minimum quantity of (CH

quantity of (CH44 + CO + H + CO + H22) ) exit exit moles moles werewere

observed to be 0.02 moles (AS1). observed to be 0.02 moles (AS1).

4.1.2. CO

4.1.2. CO22 Emission: Emission:

The CO

The CO22 emissions at 1 bar pressure and 700°C emissions at 1 bar pressure and 700°C

of the CLC fuel reactor for different inputs of OC of the CLC fuel reactor for different inputs of OC and different input feed conditions is shown in Fig. and different input feed conditions is shown in Fig. 4. It was seen that the CO

4. It was seen that the CO22 emission from the fuel emission from the fuel

reactor increase with increase in the amount of OC. It reactor increase with increase in the amount of OC. It was also observed that the product CO

was also observed that the product CO22 molesmoles

generally decrease with increase in feed CO

generally decrease with increase in feed CO22 moles moles

at constant GaCR. The maximum amount of CO at constant GaCR. The maximum amount of CO22

emission was found to be 0.99 moles for case AS1, emission was found to be 0.99 moles for case AS1, while the minimum CO

while the minimum CO22 emission was observed to emission was observed to

be 0.87 moles (CS4). be 0.87 moles (CS4).

4.1.3. Syngas ratio: 4.1.3. Syngas ratio:

The syngas ratio at 1 bar pressure and at 700°C The syngas ratio at 1 bar pressure and at 700°C of the CLC fuel reactor for different inputs of OC of the CLC fuel reactor for different inputs of OC and different input feed conditions is shown in Fig. and different input feed conditions is shown in Fig. 5. It was observed that the syngas ratio remains 5. It was observed that the syngas ratio remains constant irrespective of increase in the amount of constant irrespective of increase in the amount of OC. It was also observed that the syngas ratio OC. It was also observed that the syngas ratio generally decreased with increase in feed CO

generally decreased with increase in feed CO22 moles moles

at constant GaCR. The maximum syngas ratio was at constant GaCR. The maximum syngas ratio was found to be 5.73 (CS1), while the minimum of found to be 5.73 (CS1), while the minimum of syngas ratio was observed to be 0.25 (CS4). This syngas ratio was observed to be 0.25 (CS4). This constant syngas ratio observation is reported for the constant syngas ratio observation is reported for the first time in this paper.

first time in this paper.

4.2

4.2 Effect of GaCR Conditions and Temperature.Effect of GaCR Conditions and Temperature. 4.2.1. Net CO

4.2.1. Net CO22emission:emission:

The Net CO

The Net CO22 emissions from the CLC process is emissions from the CLC process is

used to measure the CO

used to measure the CO22 utilization and is shown in utilization and is shown in

Fig. 6. The net CO

Fig. 6. The net CO22 emission (moles) = CO emission (moles) = CO22 moles moles

(in output) - CO

(in output) - CO22 moles (in input). An increase in moles (in input). An increase in

0.00 0.00 0.02 0.02 0.04 0.04 0.06 0.06 0.08 0.08 0.10 0.10 0.12 0.12 0.14 0.14 0.16 0.16 0.18 0.18 0.20 0.20 A ASS1 1 AASS3 3 BBSS1 1 BBSS3 3 BBSS5 5 CCSS1 1 CCSS33 ( ( C C H H 4 4 + + C C O O + + H H 2 2 ) ) m m o o l l e e s s

GaCR Feed conditon GaCR Feed conditon Fig

Fig. 3-. 3- (CH(CH₄₄+ CO + H+ CO + H₂₂) moles emitted) moles emitted from the fuel reactor at 700

from the fuel reactor at 700˚˚_C._C.

S_NiO S_NiO 1.25S_NiO 1.25S_NiO 1.5S_NiO 1.5S_NiO 0.80 0.80 0.82 0.82 0.84 0.84 0.86 0.86 0.88 0.88 0.90 0.90 0.92 0.92 0.94 0.94 0.96 0.96 0.98 0.98 1.00 1.00 1.02 1.02 A ASS1 1 AASS3 3 BBSS1 1 BBSS3 3 BBSS5 5 CCSS1 1 CCSS33 N N e e t t C C O O 2 2 m m o o l l e e s s

GaCR Feed conditon

Fig

Fig. 4. 4-- COCO22moles emitted from the fuelmoles emitted from the fuel

reactor at 700 reactor at 700˚C˚C.. S_NiO S_NiO 1.25S_NiO 1.25S_NiO 1.5S_NiO 1.5S_NiO 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 4.00 4.00 5.00 5.00 6.00 6.00 7.00 7.00 AS AS1 1 AAS3 S3 BSBS1 1 BSBS3 3 BBS5 S5 CCSS1 1 CCS3S3 S S y y n n g g a a s s r r a a t t i i o o

GaCR Feed conditon GaCR Feed conditon

Fig.

5-Fig. 5- SyngSyngas ratio at 70as ratio at 7000˚C˚C.. S_NiO S_NiO 1.25S_NiO 1.25S_NiO 1.5S_NiO 1.5S_NiO

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temperature from 500 to 1200˚C

temperature from 500 to 1200˚C resulted in decrease resulted in decrease

in net CO

in net CO22 emission. At constant GaCR, increasing emission. At constant GaCR, increasing

the CO

the CO22 moles in the input moles in the input (with simultaneous(with simultaneous

decrease in H

decrease in H22O moles)O moles) decreased the net CO decreased the net CO22

emission in output. The net maximum CO

emission in output. The net maximum CO22 emission emission

found in case CS1 (3 moles H

found in case CS1 (3 moles H22O + 0 moles COO + 0 moles CO22).).

4.2.2 H

4.2.2 H22 and CO formation: and CO formation:

The H

The H22 formation increased with increase in formation increased with increase in

temperature from 500 to 1200˚C. Increas

temperature from 500 to 1200˚C. Increas e in the COe in the CO22

moles (with simultaneous decrease in H

moles (with simultaneous decrease in H22O moles)O moles) inin

GaCR ratio decreased the H

GaCR ratio decreased the H22 formation in output. formation in output.

The maximum H

The maximum H22formation was found in case AS1formation was found in case AS1

(3 moles H

(3 moles H22O + 0 moles COO + 0 moles CO22) fig. 7a. Similarly,) fig. 7a. Similarly,

increase in the process temperature led to an increase increase in the process temperature led to an increase in CO formation. Increase in the CO

in CO formation. Increase in the CO22 in GaCR ratio in GaCR ratio

increased the CO formation in output. The minimum increased the CO formation in output. The minimum CO formation found in case AS1 (3 moles H

CO formation found in case AS1 (3 moles H22O + 0O + 0

moles CO

moles CO22) as shown in fig. 7b.) as shown in fig. 7b.

4.2.3. Syngas Yield: 4.2.3. Syngas Yield:

The syngas yield is calculated by adding the H The syngas yield is calculated by adding the H22

and CO moles obtained from the process. As seen in and CO moles obtained from the process. As seen in Fig. 8, the syngas yield generally increased with Fig. 8, the syngas yield generally increased with increase in temperature from 500 to 1200˚C and for increase in temperature from 500 to 1200˚C and for constant GaCR. Increase in the feed CO

constant GaCR. Increase in the feed CO22 moles led moles led

to a decrease in syngas yield, e.g. the syngas yield to a decrease in syngas yield, e.g. the syngas yield increased from 0.09 to 0.24 moles for case AS1, and increased from 0.09 to 0.24 moles for case AS1, and the syngas yeild increased from 0.06 to 0.37 moles the syngas yeild increased from 0.06 to 0.37 moles for case AS4 and 0.06 to 0.33 for BS5.

for case AS4 and 0.06 to 0.33 for BS5.

4.2.4. NiO conversion: 4.2.4. NiO conversion:

It is assumed that the CH

It is assumed that the CH44, CO, and H, CO, and H22 are are

oxidized to CO

oxidized to CO22 and H and H22O by means of NiO in theO by means of NiO in the

CLC fuel reactor. The oxygen carrier (NiO) gets CLC fuel reactor. The oxygen carrier (NiO) gets reduced in the CLC fuel reactor. The NiO conversion reduced in the CLC fuel reactor. The NiO conversion slightly decreased with increase in temperature from slightly decreased with increase in temperature from

500-500-1200˚C. For e.g. the NiO conversion1200˚C. For e.g. the NiO conversion decreased decreased from 93.75% to 88.23% in case of AS1 while it from 93.75% to 88.23% in case of AS1 while it decreased from 93.21% to 81.57% in case of CS4 as decreased from 93.21% to 81.57% in case of CS4 as shown in fig. 9.

shown in fig. 9.

The carbon conversion at 1 bar pressure is The carbon conversion at 1 bar pressure is shown in Table 2. It was seen that as the temperature shown in Table 2. It was seen that as the temperature increased from 500

increased from 500 – – 12001200˚˚C, the carbon conversionC, the carbon conversion reached its maximum (100%). It was observed that reached its maximum (100%). It was observed that

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the 100% carbon conversion was achieved at the 100% carbon conversion was achieved at relatively low temperature for higher GaCR.

relatively low temperature for higher GaCR.

4.2.5. 100% carbon conversion in fuel reactor: 4.2.5. 100% carbon conversion in fuel reactor:

It is very important for the CLC process to get It is very important for the CLC process to get 100% carbon conversion in the fuel rector as the 100% carbon conversion in the fuel rector as the unreacted carbon can go to in air reactor and form unreacted carbon can go to in air reactor and form CO

CO22. The lowest temperature for 100% carbon. The lowest temperature for 100% carbon

conversion was found to be 900˚C in case CS1, so conversion was found to be 900˚C in case CS1, so this point was found to be the best point for process this point was found to be the best point for process operation.

operation.

Table 2- Carbon Conversion Table 2- Carbon Conversion

V.

V. ConclusionConclusion

This theoretical study was done to understand This theoretical study was done to understand the material balances for process design of direct the material balances for process design of direct CLC of rice husk. The study considered the effect of CLC of rice husk. The study considered the effect of temperature and effect of amount of oxygen carrier temperature and effect of amount of oxygen carrier in the fuel reactor using steam and CO

in the fuel reactor using steam and CO22 as gasifying as gasifying

agents in steps. It was concluded that rice husk CLC agents in steps. It was concluded that rice husk CLC using 3 moles H

using 3 moles H22O and 0 moles COO and 0 moles CO22 per mole carbon per mole carbon

at 1 bar pressure and 900˚C is the best point to at 1 bar pressure and 900˚C is the best point to operate the process as hundred percent carbon operate the process as hundred percent carbon conversion in the rice husk occurs in the process. The conversion in the rice husk occurs in the process. The results obtained in this detailed study can be used for results obtained in this detailed study can be used for

scale up of the process. Further Experimentation is scale up of the process. Further Experimentation is required for the commercialization of t

required for the commercialization of t he technology.he technology.

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